The stator bar windings of generators of a certain size are typically water-cooled. That is, water flows from an inlet coolant water header into flow passages within the hollow copper strand stator bars and then flows outwardly of the generator into an outlet coolant header for flow into a reservoir. The coolant water is supplied the windings via a closed loop system including a heat exchanger and a deionizer. Leaks in the stator windings of water-cooled generators, for example, at the brazed joints of the windings, can eventually cause insulation damage that can affect the reliability and longevity of the unit. Early detection of such water leakage enables strategic testing and repair to be scheduled during minor outages, avoiding costly surprise replacements and extended outages. If early insulation damage is not discovered, the problem can quickly compound itself, as stator components are subject to thermal shock, cycling, corrosion and mechanical vibrations. This harsh environment causes and exacerbates leaks at a variety of locations, though most commonly at series loops and other brazed connections.
It will be appreciated that in water-cooled generators, a cooling core hydrogen environment is normally maintained at a higher pressure than the coolant water flowing through the stator windings. This pressure difference, combined with stator component permeability, particularly at the brazed joints, causes a slight, barely detectable, flow of hydrogen into the coolant water under normal operating conditions even in a leak-free generator. However, when leaks actually develop, the quantity of hydrogen flowing into the coolant water increases dramatically. By continuously or periodically monitoring the leakage flow of hydrogen into the coolant water, upward trending or step increases in the volume of hydrogen leakage can be used as a reliable indicator of water leaks and the potential for electrical insulation damage.
An additional concern involves the oxygenation level of the coolant water. With proper aeration, a tenacious and protective cupric oxide film advantageously forms on the inside surfaces of the copper windings. However, when the coolant water oxygen level drops, a less stable cuprous oxide layer is formed along these surfaces. This layer tends to break away from the winding surface, sloughing off base copper and introducing particles into the system. Oxygenation of the coolant water for generators is currently provided by air exchange through a vent line from the coolant water storage tank or reservoir to the atmosphere. Unfortunately, air in this line is relatively stagnant and the typical long length of the line, upwards of hundreds of feet in some installations, makes oxygen exchange difficult. Furthermore, significant hydrogen leaks may cause a constant outward flow of gas through this line, thus totally isolating the water from fresh air.
Present on-line testing techniques typically involve a technician climbing to the location where the water storage tank vent exits the building housing the generator. A polyethylene bag is secured over the vent and flow rate is determined by timing the period required to fill the bag. A hand-held flammable gas sensor is then placed in the bag to determine hydrogen content. Such test results are marginal and limited to the time of collection. Their value is thus questionable.